About FramelessWindowHint and winId

[Wong]

Hi guys:
I got a problem that seems to be very strange. I want to write a media player with some dll, argument is window HWND. So I input QFrame::winId(). That qframe is child of qmainwindow.

MainWindow::MainWindow(QWidget *parent):
    QMainWindow(parent)
{
    this->setupUi(this);
    m_hwnd = this->viewPage->winId(); //viewPage is a QFrame
...
}

MainWindow::MainWindow(QWidget *parent):
    QMainWindow(parent)
{
    this->setupUi(this);
    m_hwnd = this->viewPage->winId(); //viewPage is a QFrame
...
}

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It works properly.
OK.jpg

but when I write like this

MainWindow::MainWindow(QWidget* parent):
    QMainWindow(parent)
{
...
    this->setWindowFlags(Qt::WindowMinimizeButtonHint | Qt::FramelessWindowHint);
    this->setAttribute(Qt::WA_TranslucentBackground, true);
    m_hwnd = this->viewPage->winId();
...
}

MainWindow::MainWindow(QWidget* parent):
    QMainWindow(parent)
{
...
    this->setWindowFlags(Qt::WindowMinimizeButtonHint | Qt::FramelessWindowHint);
    this->setAttribute(Qt::WA_TranslucentBackground, true);
    m_hwnd = this->viewPage->winId();
...
}

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It doesn't work. I don't know why...
setWindowFlags.png

I think winId() is invalid if does not have the frame. So I try to new other QFrame and setWindowFlags.

MainWindow::MainWindow(QWidget *parent):
    QMainWindow(parent)
{
...
    this->setWindowFlags(Qt::WindowMinimizeButtonHint | Qt::FramelessWindowHint);
    this->setAttribute(Qt::WA_TranslucentBackground, true);
    QFrame*  frame= new QFrame();
    frame->setWindowFlags(Qt::FramelessWindowHint);
    frame->show();
    m_hwnd = frame->winId();
...
}

MainWindow::MainWindow(QWidget *parent):
    QMainWindow(parent)
{
...
    this->setWindowFlags(Qt::WindowMinimizeButtonHint | Qt::FramelessWindowHint);
    this->setAttribute(Qt::WA_TranslucentBackground, true);
    QFrame*  frame= new QFrame();
    frame->setWindowFlags(Qt::FramelessWindowHint);
    frame->show();
    m_hwnd = frame->winId();
...
}

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It's OK, although QFrame out of the QMainWindow.
Other Widget.jpg

Now, I want custom titlebar. And get frame winId to play file...
I do not know where to begin, and unable to perceive where the problem lay.

Please, help me.
Thanks.